1. Field of the Invention
The invention relates to an operational amplifier, and more particularly to an operational amplifier with low operating voltage.
2. Description of the Related Art
Reference voltage is typically required to provide a substantially constant output voltage despite gradual or momentary changes in input voltage, output current or temperature. In particular, many designers have utilized bandgap reference circuits due to their ability to provide a stable voltage supply that is insensitive to temperature variations over a wide temperature range. These bandgap references rely on certain temperature-dependent characteristics of the base-emitter voltage of a transistor. Bandgap reference circuits typically operate on the principle of compensating the negative temperature coefficient of a base-emitter voltage of a bipolar transistor with the positive temperature coefficient of the thermal voltage. In general cases, the temperature-independent voltage is around 1.25V, and if the system cannot provide sufficient voltage, the bandgap reference circuit may cause errors or fail to operate, i.e. based on the present architecture of bandgap reference circuits, the bandgap reference circuit cannot operate at a voltage 1.2V or lower.